It is often desirable to be able to communicate between a number of stations in the same building without installing a dedicated data transmission network. One approach for achieving such a data link is to use the building's existing power line transmission network. Data transmission systems based on existing power line transmission networks are generally referred to as Power Line Carrier (PLC) systems or Power Line Data Transmission (PLDT) systems.
Most of the previous applications for PLDT systems have involved single station to single station communications. Examples of such systems include intercoms and appliance controllers. In some applications, such as appliance controllers, a number of receivers are connected to the power line, but are controlled by a single transmitter. Systems comprising multiple transmitters and multiple receivers have not been used in the past because of a number of difficulties involved in coupling the multiple transmitters and receivers (transceivers) to the power transmission line. In particular, previous systems attempting to employ multiple transceivers have had a very limited range and effectiveness because of line coupling difficulties.
A typical AC power transmission line normally has a very low impedance, on the order of one to ten ohms. The low impedance of the AC line at frequencies usable for communications causes significant difficulties with regard to the coupling and filtering of the transmitted and received signals. The transmitter and the receiver portion of a transceiver have different coupling and filtering requirements when used for communications over power lines. In general, the coupling circuit used for the transmitter should have a very low loss, while providing moderate rejection and a fairly wide bandwidth. The loss of the transmitter coupling circuit must be low because the power requirements needed to overcome loss rise very rapidly and thus quickly become unfeasible. Bandwidth and rejection are not particularly important for a transmitter, with the exception of harmonic rejection and rejection of intermodulation signals. The receiver portion of the transceiver can withstand a fairly high loss factor, on the order of 20 dB, but the rejection should be as high as possible. The filter loss can be high because the signal to noise ratio of the received signal is limited by the noise present on the power line.
Previous coupling circuits for connecting transceivers to power transmission lines typically employ tuned tank circuits comprising a capacitor connected to the winding of a transformer. This type of coupling circuit defines a bandpass filter having a relatively low loss and moderate rejection. While this circuit is suitable for use by the transmitter, it does not meet the coupling requirements of the receiver. Systems using a single circuit of this type for coupling the signals of both the transmitter and the receiver have, therefore, been ineffective for the reasons discussed above.